1. Field of the Invention
The present invention relates generally to bicycle front derailleurs used to shift a bicycle chain from one chain ring to another chain ring on a bicycle crank set. More particularly, this invention relates to a novel six link front derailleur.
2. Description of Related Art
Bicycle front derailleurs are well known in the art. A front derailleur is used to shift gears on a bicycle crank set that has at least two and perhaps three different sized chain rings. Generally speaking, a front derailleur includes an attachment mechanism for securing the front derailleur to the bicycle frame, a chain guide that surrounds the bicycle chain at all times, and a movement mechanism that selectively positions the chain guide over a given chain ring, consequently shifting the position of the bicycle chain to the given chain ring. The movement mechanism of a front derailleur is typically controlled by a shifting cable attached to a shift lever or grip-shifter that provides tension against a biasing element such as a spring that urges the front derailleur to a particular starting position.
The attachment mechanism of a bicycle front derailleur may be a circular clamp used to surround a bicycle frame member such as a seat tube or other frame member. Alternatively, the attachment mechanism may be bolt and bracket configured for mating with a corresponding bracket that has been brazed, welded, or otherwise affixed to a bicycle frame member.
The chain guide of a front derailleur may be of the conventional inner and outer guide member variety that laterally exerts a force against the enclosed chain, causing the chain to be displaced laterally and eventually catch upon the teeth of the adjacent chain ring during a shift. U.S. Pat. No. 4,734,083 to Nagano is an example of a conventional front derailleur with conventional inner and outer guide members forming the chain guide. More recently, chain guides having improved shifting characteristics, i.e., that lift up and pull down on the bicycle chain during shifting have been developed and are disclosed in U.S. Pat. No. 6,454,671 to Wickliffe.
There are many variations on conventional movement mechanisms for bicycle front derailleurs. For example, U.S. Pat. No. 4,734,083 to Nagano describes a relatively common four link movement mechanism. U.S. Pat. No. 6,454,671 to Wickliffe discloses a linear movement mechanism for a front derailleur, based on dual guide rods. U.S. Pat. No. 5,649,877 to Patterson discloses a linear movement mechanism for a front derailleur, based on a single piston mechanism. Most conventional movement mechanisms rely on spring biasing and opposing control cable tension. However, alternatives to cable driven front derailleurs have been proposed. For example, a pneumatic actuation system for driving a front bicycle derailleur movement mechanism has been proposed in U.S. Pat. No. 6,159,119 to Nakamura et al. Additionally, an electrical motor for driving for driving a front bicycle derailleur movement mechanism has been proposed in U.S. Pat. No. 7,331,890 to Ichida et al.
However, there is a need in the art for a robust bicycle front derailleur that is capable of significant vertical displacement of a chain guide. It would also be useful if an embodiment of a front derailleur included a horizontal adjustment feature for positioning the chain guide relative to the chain rings. It would be further useful if an embodiment of a front derailleur included a mechanism for preventing a bicycle chain from falling inside of the inner chain ring during an over-shift condition. It would be useful if an embodiment of a front derailleur were configured with a chain guide that could lift up and pull down on a bicycle chain during shifting.